Throughout history people puzzled over what exactly determined a whether a baby would be a boy or a girl. Theories tended to range from the woman's diet, to the ambient temperature of the room. Until one woman, studying mealworms, finally determined what was the difference between little girls and little boys.

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Henry VIII would probably have had Nettie Stevens killed. He lived at a time when it was often considered the lady's job to make sure a king would get a male heir. (He also had a position and temperament which ensured that anyone he consulted would be very clear about it being the wife's fault.) In many different times it was considered both right and convenient that a woman determined the sex of the baby, and special diets and exercises were doled out to get the right kind. At other times, sex determination was attributed to the man, with people thinking a father could "get" a boy by everything from his nightclothes to the temperature of his semen. (Though how they managed a variation in semen temperature is anyone's guess.) But into the 1900s, with people aware of Mendel, genetics, and chromosomes, people were still unsure of why some babies turned out to be boys and some to be girls.

Enter Nettie, a bookworm and academic who, at 35, was sick of teaching and wanted to go into research. She enrolled at Stanford, working towards her master's degree in biology. As she studied, she slowly switched her focus from physiology to heritability. The subject most favored by scientist in those days was the mealworm. It was common, and easy to cultivate. Stevens looked at the mating process, and noticed that all unfertilized eggs were exactly the same. Mealworm sperm cell chromosomes, however, came in two different sizes. There was a large chromosome, much like the one in the female egg, which Stevens called an X chromosome. Meanwhile, there was a small chromosome that Stevens dubbed Y. A little research showed that all eggs fertilized with the Y chromosome grew up to be male mealworms. The double X chromosome, however, guaranteed a distaff worm. She came up with the X,Y model for sex selection.

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She published a paper on the subject in 1905. That same year, a scientist named Edmund Wilson published a similar finding. The organism he worked with, however, was missing a chromosome, and so he attributed the difference in sexes to a much less quoted X,O model. On the strength of her paper, and subsequent research work, Stevens began working as a full-time research scientist at 39. A position was even created for her at Bryn Mawr. Sadly, she died of breast cancer at 51. Still, whenever we see an XX, XY icon, or make mention of the two most quotable chromosomes, we're citing her work.